The objectives of this proposal are to determine the molecular mechanisms which control the expression of histone mRNA levels during the mammalian cell cycle. The signals which regulate histone mRNAs are tightly coordinated with the signals which regulate cell growth. Many of the common chemotherapy drugs have rapid, profound effects on histone mRNA metabolism, as a result of the tight coupling of DNA replication and histone protein synthesis. The 3' end of histone mRNA is responsible for much of the regulation of histone mRNA levels both by regulating 3' processing and regulating histone mRNA half-life. The goal of this proposal is to clone the trans-acting factors involved in mediating the functions of the 3' end of histone mRNA and to understand the molecular mechanisms which govern their function. The specific aims are to: 1. clone the cDNA for the 45 kD protein, SLBP, which is bound to the 3' end of histone mRNA on the polyribosomes; 2. characterize the functional domains of the SLBP, with respect to RNA binding and regulation of RNA stability;. 3. Determine whether the SLBP also participates in the 3' processing reaction, or whether there is a separate factor which also binds the histone 3' end in the nucleus; 4. Using antibodies against the SLBP determine the possible modifications of the protein which are involved in coupling histone mRNA metabolism to DNA replication, and in particular whether cyclin-dependent kinases play a role in regulating SLBP function. 5. Using in situ methods, determine whether the two clusters of histone genes are colocalized with the U7 snRNP in the nucleus as a function of the cell cycle.